The invention relates to a device for biological digestion of sewage with rotationally symmetric rotatable growth surfaces preferably made of plastic, namely so-called immersion percolating filters, which are immersed in the sewage as they are made to rotate by a drive.
In known digestion systems of this type, the immersion percolating filters used are approximately half-immersed in the sewage. The bacteria and micro-organisms required for digestion grow on their growth surfaces.
If the immersion percolating filter is used to decompose the organic carbon contained in the sewage, the micro-organisms on the growth surface, when they are dipped into the sewage, take in the organic carbon as "food" and take in the atmospheric oxygen for breathing when they surface.
According to the invention, ammonia nitrogen is to be converted into nitrate nitrogen, something which also requires oxygen. When the sewage is heavily contaminated with organic matter, it may be necessary for the growth surfaces of the immersion percolating filter to rotate not in a normal air atmosphere when they are outside the sewage but in an oxygen-nitrogen atmosphere containing for example over 60% oxygen. When sewage containing a great deal of ammonia is being oxidized, it may also be economical to increase the O.sub.2 concentration. According to the invention, this method step is also carried out.
In addition, the object of the invention also makes it possible to carry out nitrification.
When the immersion percolating filter process is used for nitrification, the growth surfaces must pass through an oxygen-free atmosphere which contains, for example, fermentation gas or similar gases which can serve as electron donors.
The production of a special gas atmosphere is accordingly decisive for the efffectiveness of the biological digestion process in question.
Another decisive factor for the efficiency of the immersion percolating filter is the residence time in the tanks for a given growth area of the immersion percolating filter.
The biological processes for elimination of organic carbon, oxidation of the ammonia into nitrate, and denitrification of the nitrate into gaseous nitrogen require different residence times with a given growth surface area.
According to the invention, this problem can be solved with a device in which a gas-tight container is provided to eliminate organic carbon, oxidize ammonia into nitrate, and denitrify nitrate into gaseous nitrogen, in which container at least one immersion percolating filter cylinder having several immersion percolating filter plates adjacent to each other is mounted preferably above the maximum sewage level, is drivable, and has an adjustable height. The sewage level is adjusted according to the circumstances, namely according to the type and composition of the sewage to be digested. This container can be fed without difficulty at any time with different types of gases in succession.
The immersion percolating filter cylinder inside the container is also designed such that it has a supporting ring on one side and a drive ring on the other side, which rings are connected to each other by support rods distributed uniformly around the periphery, between which rods the growth surfaces are attached. In this way, the growth surfaces can consist of rigid plastic plates or plastic film. The latter must, of course, be suspended in a spring-tensioned manner. Because of this design, the growth surfaces preferably have a smaller diameter than the rings.
The drive ring consists of two sheet iron rings on edge, which rings are connected to each other by stud bolts and which hence are at a distance from each other such that a drive pinion mounted above the maximum sewage level causes the immersion percolating cylinder to rotate slowly when the pinion rotates because its teeth engage the stud bolts.
Immersion percolating filter cylinders of this type have no center shaft as do most of the previously known immersion percolating cylinders. The drive does not engage the center of the cylinder but its periphery. This facilitates driving. This is especially the case when the cylinder has been resting for a long time and the growth layer of the biological mat extending out of the water has dried thereby causing an imbalance. With the hitherto usual designs, the shafts or chain wheels frequently broke in such cases or the chains snapped.
The drive pinion advantageously engages the stud bolts at the apex of the drive ring and is located outside or inside the ring.
The outer rings of the immersion percolating filter cylinder each run on two flanged guide rollers located inside the rings and above the maximum sewage level. Because of the immersion percolating filter cylinder's own weight and the weight of the biological material growing on its growth surfaces, the rings of the cylinder fit closely abut the flanged guide rollers so that the cylinder would be sufficiently reliably mounted by itself. To enhance the safety and for purposes of perfect lateral guidance of the immersion percolating filter cylinder, however, a third flanged guide roller could be mounted on the inner wall of the immersion percolating filter container.
All the rotating parts cooperating with the immersion percolating filter cylinder can be removed in the outward direction for easy disassembly for repair and maintenance work, and are screwed onto the container from the outside with a gas-tight flange. These rotating parts are the flanged guide rollers, the pivot bearings, and a drive motor to drive the pinion, and they are designed and arranged such that rotating parts can move only inside the container.
If any repairs become necessary, or when the immersion percolating filter cylinder is being removed from the tank, the roller bearings, the pinion, and the drive motor are removed. The immersion percolating filter cylinder is now resting on the drive and bearing ring on the bottom of the immersion percolating filter container without harming the growth surfaces.
Since the container must be closable in a gas-tight manner, if necessary, the inner space of the container above the sewage level can be made to communicate with the outside by means of sealing fluid in a syphon and via a pressure relief valve, which valve relieves the pressure when it exceeds about 500 mm water column.
In order to feed the optimally adapted type and quantity of gas into the gas space of the container for the digestion process being carried out, a gas pipe leads into the inner space of the container above the sewage level said pipe being fitted with a gas pressure adjustment valve.
The air leaving the immersion percolating filter container is sealed off by a pressure valve which consists of a U tube with a sealing liquid. This valve is set as required for a range of 20 to 500 mm water column.
The sewage flows in at a level slightly higher than the lowest sewage level expected, while the outlet pipe is connected with an outlet located slightly below the sewage inlet, which outlet can be at any level between the lowest and the highest of the expected sewage levels. The residence time can be optimally adapted to the requirements of the occasion by changing the sewage level.
It is also possible to enlarge the immersion percolating filter cylinder. For this purpose, at least one more immersion percolating filter cylinder is flanged to a supporting ring of an immersion percolating filter cylinder which cylinder has the drive ring on the other side, said second cylinder having only supporting rings. The thus enlarged cylinder is mounted in a larger container.